powerpc: Remove power8 strcasestr optimization

Similar to strstr (1e9a550b), power8 strcasestr does not show much
improvement compared to the generic implementation.  The geomean
on bench-strcasestr shows:

            __strcasestr_power8  __strcasestr_ppc
  power10                  1159              1120
  power9                   1640              1469
  power8                   1787              1904

The strcasestr uses the same 'trick' as power7 strstr to detect
potential quadradic behavior, which only adds overheads for input
that trigger quadradic behavior and it is really a hack.

Checked on powerpc64le-linux-gnu.
Reviewed-by: DJ Delorie <dj@redhat.com>